Canning – The Science Behind It

Food preservation began in a time when there were no refrigerators and it was therefore difficult to store perishable foods during off season. The solution was to store raw or cooked food in a clean glass jar with a lid, and to sterilise the contents by heating up the jars in a water bath. During this process, the liquid inside the jar forms steam which then replaces the air and pressure builds within the jar. This can take between 30 and 60 minutes. Once cooled, the jar forms a kind of vacuum and a seal just below the lid, allowing it to keep for years.

This method was made popular in 1810, when Napoleon offered a cash prize to the person who could figure out a way of preserving nutritious foods for his troops at war. In principle, almost all types of fruit and vegetables are suited to this method, and also liquids such as soups and tomato sauce are ideal for canning. It is even possible to bake cakes and breads in such jars. The only food that it does not suit is dairy products as these require higher temperatures.

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If you don’t have a big pot suited to the purpose, canning can also be done in an oven at 175°C. Simply fill a deep baking tray with water and gently place the closed glass jars inside. After 30 minutes, switch off the oven and leave the jars sitting there for a few minutes before taking them out to fully cool.

Canning – The Right Utensils

This method of canning can also be called bottling. The Weck jar is ideal for canning. It has a flat, glass lid and is sealed by a rubber seal and two steel tension clips. A variation on this is the glass terrine jar with a clip that attaches the lid to the jar itself. In addition to this, you will need a large pot or a special canning pot and a thermometer.

Canning – Preparation

Canning is a method for keeping perishable food for longer. In order to ensure that no bacteria gets into the preserved food, hygiene in the kitchen and a very careful approach are the two basic prerequisites for successful food preservation. Firstly, jars must be cleaned and sterilised before use. Next, examine your utensils – are the rubber rings porous or is the glass cracked? If both look like they are in good condition, then you are ready to go.

Wash out the jars and lids with hot water by placing them in a pot of cold water and heating it up gradually. When the water has boiled, take the pot off the heat and once cool enough to handle, remove the jars. Do the same for the rubber bands except add a drop of vinegar to the water. Note: Never let your utensils come into contact with washing up liquid as this will prevent them from closing fully. Always clean your hands before handling the utensils. The same rules apply to your accessories such as your filling funnel.

How to prepare the food

The vegetables or fruit require a thorough preparation: clean them well and remove any rotten pieces. Overripe products should not be preserved as they can begin to ferment in the jar. Slice the food into smaller pieces – the exact size depends on your preference. Harder vegetables such as kohlrabi, pumpkin or carrots can be quickly blanched in salt water in order to give them a softer texture.

Begin preparing the broth - fruit and vegetables must be preserved in liquid, otherwise no steam is created and therefore no vacuum. To sweeten the fruit mixture, add water with some sugar, and to make the vegetable mix sour, add water with some vinegar or salt. When combining for example vegetables with onions or spices, it is best to insert them in alternating layers so that the flavours are distributed equally.

Canning – Water Bath

Now you can place the jars in the water bath. The water in the pot should reach about two thirds of the height of the jar. If you insert cold contents into the jar, it is best to also place the jar in cold water and slowly heat it. The canning time begins as soon as air bubbles begin to rise in the jar – then you can lower the temperature.

When the cooking time is up, carefully remove the jars from the water bath and let them cool on a tea towel. Only when the jars are completely cold, can you remove the clamps and make sure that the lid is totally sealed. Store the cooked, preserved goods in a dark pantry, cellar or storage cabinet.

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